4.5 Population Dynamics

This module covers how populations are managed, how they grow and also how organic matter is broken down (The role of bacteria in the nitrogen cycle.

Population dynamics is the study of population size in the long and short term. First familiarize yourself with the vocab under ecosystem dynamics, then read this, and refer back to the definition as and when you need to.

COMPETITION:In ecology, there are 2 types of competition:1. Interspecific competition, the competition between members of different species, for example, 2 species trying to occupy the same niche will be competing for food, water, shelter and other resources. The species most suited to that niche will win (in general) and dominate the community in that habitat. This can be demonstrated by growing 2 populations of Paramecium:P. aurelia and P. caudatum when grown in seperate flasks reach similar population levels, however, when grown together, P. aurelia dominates the habitat and P. caudatum will be wiped out, or present in very low number.

2. Intraspecific competition, the competition between members of the same species for food, water, shelter etc. This type of competition is more prominent as the members are fighting for exactly the same resources as each other. When a population starts to increase, so does intraspecific competition up until the carrying capacity is reached. As the population declines, so does intraspecific competition. This generally happens as a cycle through time as demonstrated in the graph below:

In harsh environments, it is the abiotic factors (e.g. temperature, rainfall, pH etc) that governs the species that live in that habitat.In favourable environments, it is the biotic factors (competition) that defines which species occupy the available niches.

PREDATION:The population of predators and prey are inter-related. This is best explained through pictorial form:

As you can see, there is a delay/lag between the peak number of predators and prey. This is due to the time taken for offspring to be produced.The process can be described in a simple cycle:Prey increases -> Predator increases -> Prey decreases -> Predator decreases (in terms of numbers).The amount of predators vs prey is a low ratio due to the fact that ~90% of all energy is lost between trophic levels.

POPULATION GROWTH:When a new species is introduced into an environment, there is a characteristic growth/time curve for most species called: Sigmoid growth curve. This curve has four phases detailed below the curve.

Lag phase - Low increase in population numbers due to organisms 'acclimatising' by:- Having to find food- Synthesizing enzymes- Finding a mateExponential Phase - Rapid increase in population due to:

PEST CONTROL:Pests are organisms that reduce the yield of a crop (refer to definitions)There are 2 forms of pest control in the syllabus; Chemical & biological:

1. CHEMICAL CONTROL:

(Studying insecticides) Insecticides should ideally target as few pests as possible (specific), they should be biodegradable (Non-persistent) and finally, Toxins should not accumulate in predators of the pest causing poisoning (Bioaccumulation).

- Pest population can develop resistance against the drug due to the selective pressure on the population- Some pesticides may kill animals as their food may be contaminated with the insecticide.- Long term exposure to insecticides may be bad for human health

2. BIOLOGICAL CONTROL:A predator is introduced into the pest's ecosystem/habitat to kill the majority of the pests.

ADVANTAGES- Highly specificity - No development of resistance in the population- No environmental contamination- Used safely in green houses- Provides long term control.

DISADVANTAGES- Frequent input is needed to attain a population balance.- Biological control agents are slow to build up in numbers when reacting with a sudden increase in population of the pests- Specific only to a limited species.- Does not fully eradicate all the pests- Success requires a lot of research and skill (expensive)- Detailed knowledge of the predator's life cycle is required to make sure it won't become a pest in future.

DEFINITIONS:ECOSYSTEM DYNAMICS:Species - A group of organisms that share similar features and can reproduce to make fertile offspring. e.g. oak treeHabitat - The physical space where an organism lives. e.g. the grass a snake lives inPopulation - The total number of organisms of one species within a habitat. e.g. the number of blue tits in a gardenCommunity - The total number of organisms of all species within a habitat. e.g. the sum of all animals and plants etc in a woodlandEcosystem - The community of organisms, their inter-relationships (biotic factors) and their interactions with their environment (abiotic factors) that make up a self-contained system whcih is self supporting in terms of energy flow e.g. a woodNiche - A part of a habitat which is inhabited by an organism and it's role within the ecosystem e.g. a worm's habitat & role.Carrying capacity - The maximum population that a particular environment can support. Biotic factor - A factor created by any living component in an environment in which the effect of that factor affects the life of another organism. e.g. competition for resourcesAbiotic factor - A chemical/physical factor that affects the life of an organism e.g. firesDensity independent factor - Factors that affect all members of a population irrespective of size of population. e.g. firesDensity dependent factor - Factors that affect all members of a population dependent on the size of that population. e.g. food availability.

COMPETITION:Interspecific competition - The competition between 2 different species for the same resources. e.g. P. aurelia vs. P. caudatumIntraspecific competition - The competitions between organisms in the same species for the same resources. e.g. P. aurelia vs. P. aurelia.Predator - an organism that preys (eats) upon another organism. e.g. an owl (predator) eating a mouse (prey).Prey - an organism that is predated upon e.g. mouse (look at above def)

PEST CONTROL:Pest - any organism (animal, plant or microbe) that reduces the yield of the crop e.g. by causing disease, competing for resources, feeding on the crops or spoiling the harvest.Specific - Targets a few pests.Non-persistent - Biodegradable in the environment.Non-Bioaccumulate - Toxins are not passed along in the food chain.

NUTRIENT CYCLES:In the environment, matter cycles between the biotic (organic) environment and the abiotitc (inorganic) environment. CO2, N2 and H20 are fixed from the abiotic environment by producers and built into complex molecules becoming organic molecules hence part of the biotic environment.We study just 2 cycles; the carbon cycle and the nitrogen cycle (the latter being more interesting..)

CARBON CYCLECO2 is fixed into complex organic ions by photosynthesis. They produce: Starch, Cellulose, Glucose, Sucrose, Amino Acids, DNA, RNA, ATP and lipids. Plants will respire some of the carbohydrates releasing CO2 back into the atmosphere. Animals then eat the biomass of the plants respiring to produce CO2. Finally Saprophytes decompose the biomass of other organisms when they die, They break down the complex organic molecules to produce simple inorganic ions.

2 types of decomposers:Saprophytes- Microbes that live on detritus by secreting enzymes and digesting the biomass by extracellular digestion (e.g. Fungi/Bacteria)Detrivores - Animals that eat and digest detritus. They break it into much smaller pieces for enzyme action making it more accessible to saprophytes.

NITROGEN CYCLE:Nitrogen is the basis of amino acids, ATP, nucleotides, NAD/NADP etc. Nitrogen is fixed from N2 in the atmosphere by bacteria into NH3 which dissolves to form ammonium ions (NH4^+). This is then converted into nitrite ions (NO2^-) then into nitrate ions (NO3^-)

Types of nitrogen fixing bacteriaAzotobacter - Lives in the soil, fixing N2 to NH3Rhizobium - Lives inside of root nodules (mutualistic relationship) fixing N2 to NH3.(N.B. Nitrogen can also be fixed by the haber process and when lightening strikes)q

Types of Nitrificating bacteriaNitrification is the processs by which NH3 is oxidised into NO2^- then NO3^-.Nitrosomonas - oxidise ammonia (NH3) into nitrite ions (NO2^-)Nitrobacter- oxdise nitrite (NO2^-) into nitrate ions (NO3^-)

PUTREFACTIONSaprophytes decompose nitrogenous organic matter (proteins etc) by extracellular digestion removing the amino group from amino acids this is then followed by the addition of a hydrogen to from ammonia (NH3).

DENTRIFICATIONAnaerobic dentrifying bacteria convert NO3^- to N2, this is a loss of useful 'nitrogen' from the soil. Dentrification occurs when soil because :- Waterlogged- CompactedThis is the reason farmers plough they're land as it increases the oxygen content and ensures there is plenty of air spaces. When plants are harvested, they take their nitrogenous compounds with then hence farmers have to use fertilisers to replace the lost nitrates. Nitrates are also soluble in water so excessive rain can wash away the nitrates.